Ultrasonic cleaning systems are well known and are currently used in a wide variety of applications to clean various types of substrate surfaces (e.g. surfaces on semiconductor wafers, component parts with complex surface geometries, electronic devices, etc). In a typical arrangement, a cleaning system includes a tank that holds a fluid medium such as an aqueous solution. The aqueous solution generally includes additives such as surfactants and detergents that enhance the cleaning performance of the system.
In a simple ultrasonic system, a transducer which is typically an electrostrictive or a magnetostrictive transducer, is provided to generate high frequency vibrations in the cleaning tank in response to an electrical signal input. As used herein, the term ‘ultrasonic’ and its derivatives means sonic waves having a wave frequency above approximately 15 kHz and includes both the traditional ultrasonic spectrum which extends in frequency from approximately 15 kHz to 400 kHz and the more recently used megasonic spectrum which extends in frequency from about 500 kHz to about 3 MHz.
Once generated, the transducer vibrations propagate through the fluid medium in the cleaning tank until they reach the substrate to be cleaned. More specifically, the vibrations generate an ultrasonic wave within the fluid medium wherein each point along the wave oscillates within a pressure range between a pressure maximum (compression) and a pressure minimum (rarefaction). When the pressure minimum is below the vapor pressure of the fluid medium, the ultrasonic wave can cause cavitation bubbles to form in the fluid medium.
As a result of the time-varying pressure field created by the transducer vibrations, cavitation bubbles form at sites in the fluid medium when the site pressure drops below the vapor pressure of the fluid and approaches the pressure minimum. These cavitation bubbles subsequently collapse (i.e. implode) as the site transitions from the pressure minimum to the pressure maximum. During bubble implosion, surrounding fluid quickly flows to fill the void created by the collapsing bubble and this flow results in an intense shock wave which is uniquely suited to substrate surface cleaning. Specifically, bubble implosions that occur near or at the substrate surface will generate shock waves that can dislodge contaminants and other soils from the substrate surface. The implosion of the many bubbles near the substrate surface over time results in an intense scrubbing action that is very efficient in cleaning devices such as memory disks, semiconductor wafers, LCD devices and the like.
In almost all cleaning applications, it is important to control the cavitation energy. If an insufficient amount of cavitation energy is provided, undesirably long process time may be required to obtain a desired level of cleaning, or in some cases, a desired level of surface cleaning may not be achievable. On the other hand, excessive cavitation energy near a substrate having delicate surfaces or components can cause substrate damage. Examples of substrate damage include the formation of pits and/or craters on the substrate surface. One factor that affects the size of the cavitation bubbles and the corresponding cavitation energy is the frequency of the ultrasonic wave. Specifically, at higher wave frequencies there is less time for the bubble to grow. The result is smaller bubbles and a corresponding reduction in cavitation energy.
Another factor that affects cavitation energy is the intensity of the ultrasonic wave (i.e. wave amplitude) produced by the transducer(s). In greater detail, higher wave intensities cause each point along the wave to oscillate over a larger pressure range (between rarefaction and compression), which in turn, produces larger cavitation bubbles and larger cavitation energy. Thus, there is a direct correlation between the intensity of the ultrasonic wave, the pressure range that the fluid medium oscillates between, and cavitation energy. The pressure range that the fluid medium oscillates between can be characterized as having a pressure peak relative to the ambient pressure in the fluid medium when ultrasonic waves are absent. Accordingly, pressure peak can be used as a suitable measure of cavitation energy. One additional factor that should be considered is the fact that the intensity of an ultrasonic wave will decrease as the wave propagates through the fluid medium. Thus, cavitation energy is generally a function of distance from the transducer. As a consequence, portions of a substrate that are located at different distances from an ultrasonic energy source will experience different levels of cavitation energy. As a result, it has been somewhat challenging to uniformly clean a substrate having a complex surface geometry. As used herein, the term ‘complex surface geometry’ means any surface that is not substantially flat.
With the above in mind, an array of transducer elements can be used to direct ultrasonic energy to a localized area on a substrate surface. Specifically, the phases of the electrical signals driving each transducer element can be selectively delayed to cause a localized area on the substrate surface to receive relatively intense ultrasonic energy while surrounding areas on the substrate surface receive significantly less ultrasonic energy. The localized region of intense ultrasonic energy is a result of interference (constructive and destructive) between the ultrasonic waves generated by the transducer elements. By adjusting the phase delays over time, the region of intense ultrasonic energy can be electronically scanned through space. For example, U.S. Pat. No. 6,554,003, which issued to Birang et al. on Apr. 29, 2003, discloses a transducer array and a method for adjusting the energy waves produced by each transducer to scan an ‘energy wave maximum’ along a surface of a thin disc. However, the present invention recognizes that merely scanning an ‘energy wave maximum’ along a surface does not necessarily ensure that each point on the surface experiences the same cavitation energy. For example, portions of a substrate that are located at different distances from the transducer will experience different levels of cavitation energy because the intensity of an ultrasonic wave decreases as it propagates through the fluid medium. Also, and importantly, portions of a substrate that are located at different angles from the transducer will experience different levels of cavitation energy because the intensity of an ultrasonic wave is a function of steering angle (or focusing angle). Thus, an undesirable variation in cavitation energy is present when an “energy wave maximum” is scanned along a flat surface, and it is to be appreciated that an even greater variation in cavitation energy occurs for substrates having complex surface geometries that are cleaned by scanning an “energy wave maximum” along the substrate surface.
In light of the above, it is an object of the present invention to provide systems and methods suitable for the purposes of directing ultrasonic energy to establish a target pressure peak and thus a target cavitation energy at a location on a substrate surface to efficiently clean the surface. It is another object of the present invention to provide systems and methods for uniformly cleaning a substrate surface including the surface of a substrate having a complex surface geometry.